1. Technical Field
The present invention is directed to a communication device, a multi carrier transmission system, a communication method, and a recording medium storing a communication program, which may be applicable to an xDSL (x Digital Subscriber Line) (where, x is the general term of A, S, and V) that enables high rate transmission of a few M bits/second through a metallic cable such as a telecommunication line.
2. Background Art
In recent years, xDSL technologies have been in the spotlight, which enable high rate transmission of a few M bits/second through a metallic cable such as a telecommunication cable. For example, xDSL includes ADSL (Asymmetric Digital Subscriber Line), SDSL (Symmetric Digital Subscriber Line), HDSL (High bit-rate Digital Subscriber Line), and VDSL (Very high bit-rate Digital Subscriber Line). These xDSL technologies may be differentiated depending on transmission speed, symmetry/asymmetry of speed, and the like. The above listed technologies are generally referred to as an “xDSL” technology.
An xDSL service is called “best effort type service”, and a transmission speed of xDSL varies with an environment condition such as the transmission distance of a communication line or noise.
Generally, in an environment condition which has a short communication line and a small amount of noise, the transmission speed increases. On the contrary, in an environment which has a long transmission distance and a large amount of noise, the transmission speed decreases.
An xDSL service employees a DMT (Discrete Multi Tone) method as its modulation scheme.
The DMT method performs an initialization training process and measures the transmission distance of the communication line or SNR (Signal to Noise Ratio) of each carrier before a modem initiates a communication. And, based on the measured SNR of each carrier, the DMT method calculates a bit rate to be arranged to each carrier and determines a final transmission speed based on the calculated bit rate.
A system as shown in FIG. 1 may be taken as an example of a general multi carrier transmission system using an xDSL technology. Hereinafter, the general multi carrier transmission system will be described with reference to FIG. 1.
In a construction of the general multi carrier transmission system as shown in FIG. 1, an OLT (Optical Line Terminal) intra-station device 1 is connected to a VDSL home-side device 3 via a remote combined device 2. And, a VDSL station-side device 4 is connected to a VDSL home-side device 5 via a communication line 12. The remote combined device 2 includes an ONU (Optical Network Unit) and the VDSL station-side device. The ONU is connected to the OLT intra-station device 1 through a communication line 10. The VDSL station-side device 4 is connected to the VDSL home-side device 3 through a communication line 11.
Noise may be added into the communication line (12) in the multi carrier transmission system shown in FIG. 1, as shown in FIG. 2. An example of the noise may include cross-talk noise. Thus, each of units including the multi carrier transmission system includes a function that measures noise created on the communication line and adjusts an SNR value based on the measured noise.
The noise may be fluctuated over a long term period as shown in FIG. 3(a). Or, the noise may take place during transmission as shown in FIG. 3(b).
In such a case as shown in FIGS. 3(a) and 3(b), the adjusted SNR value suffers from lack of a margin value. As a consequence, a communication error or link freezing takes place, which may lead to deterioration in line quality. FIG. 3(a) depicts a case where noise has been measured for a long time. For example, this long-term noise measurement may give rise to a predetermined number of different results (for example, 100 different results) out of the total measurement results (for example, 400 results). FIG. 3(b) depicts a case where different measurement results, for example “the measurement results caused by an error or link freezing”, take place out of the overall measurement results.
It has been suggested to apply virtual noise to known noise whose occurrence is anticipated to secure an SNR margin value for purpose of redundancy as shown in FIG. 4 in order to suppress the occurrence of communication errors or link freezing. FIG. 4 illustrates a case where virtual noise is added to some noise to yield noise used for calculation of bit allocation and an SNR margin value is secured for redundancy with respect to the noise used for calculation of bit allocation.
However, a high capacity memory is required to store the amount of noise (noise value) virtually applied over the overall carriers for the application of virtual noise as shown in FIG. 4. Accordingly, it is required to store the amount of noise for plural points (break points) as shown in FIG. 5.
FIG. 5 illustrates a case where each of plural break points is allocated to each of plural frequencies that are distributed at the same interval (for example, 1 MHz), respectively, in a one-to-one correspondence manner, wherein the amount of noise at the allocated break points is stored in a memory. That is, the amount of noise marked with “•” is stored in the memory as shown in FIG. 5.
However, if each of plural break points is allocated to each of plural frequencies that are distributed at the same interval (for example, 1 MHz), respectively, in a one-to-one correspondence manner, and the amount of noise at the whole break points is stored in the memory, the amount of noise may be stored in the memory even with respect to a frequency band that is not used for communication.
As a consequence, the case where the break points are allocated to the frequencies at the same interval as shown in FIG. 5 may give rise to a result of storing the unnecessary amount of noise in the memory. Accordingly, there is a need of suppressing unnecessary break points.
There is a patent application filed prior to the filing of this patent application, which discloses a technology of carrying out effective multi carrier transmission even in a case where some noise whose occurrence is not anticipated takes place (for example, refer to Japanese Patent Application Laid-Open (JP-A) No. 2006-165978).
Also, there is a document that discloses a technology of implementing stable communications in a noise pattern environment that is prone to cause unstable communications (for example, refer to Japanese Patent Application Laid-Open (JP-A) No. 2006-203380).
Furthermore, there is a document that discloses a technology of improving transmission efficiency in an ADSL (Asymmetric Digital Subscriber Line) network (for example, refer to Japanese Patent Application Publication (JP-A) No. 2005-534256).
JP-A No. 2006-165978 and JP-A No. 2006-203380 also disclose a technology of achieving stable communications. JP-A No. 2005-534256 also discloses allocating plural break points. However, no JP-A No. 2006-165978, JP-A No. 2006-203380 and JP-A No. 2005-534256 disclose suppressing unnecessary break points and its necessity.